CHEMICAL WEED CONTROL

Chemicals that are used to kill plants or weeds are called herbicides.

Soil application

Soil surface application: Herbicides are usually applied to soil surface to form a uniform herbicide layer. The applied herbicides, due to their low solubility may penetrate only few centimeters into the soil. Weeds germinating in the top layers are killed due to incidental absorption of herbicides.
eg. triazines, ureas and anilide

Soil incorporation: Some herbicides are applied to soil surface and incorporated into the soil either by tillage or irrigation for their effectiveness.
eg. volatile herbicides viz., aniline and carbamate

Sub-surface application: Perennial weeds like Cyperus rotundus and Cynodon dactylon are controlled by injecting herbicides to the lower layers of the soil at several points.

Band application: Herbicides are applied as narrow bands over or along the crop row. The weeds in between the crop rows can be controlled by intercultivation or band application of herbicide. This method is useful where labour is expensive and intercultivation is possible.
eg. Weeds in maize can be controlled effectively by spraying atrazine on seed row at the time of sowing.

Foliar application

Blanket application: Application of herbicide over the entire leaf area. Selective herbicides are applied by this method.

Directed Application: Herbicide are applied directly to weeds between crop rows, avoiding the crop foliage. Care is taken to avoid spray fluid falling on the crop.
eg. Late weeds in cotton can be controlled by spraying non selective herbicide by directed spray.

Spot application: Herbicides are applied or poured on small patches of weeds, leaving the relatively wee free patches untreated. It minimizes the herbicide usage per unit area.

Benefits of chemical method:

Herbicides can be applied for weed control in crop rows and where cultivation is impossible.
Pre-emergence herbicides provide early season weed control.
Cultivation & manual methods of weed control may injure the root system.
Herbicides reduce the need for pre-planting tillage. They are extremely useful in minimal\zero tillage.
Herbicides can control many perennial weed which cannot be controlled by other methods. Eg: Cyperus sp.,

eg: Rice ecosystem

Apply pendimethalin 1.0kg/ha on 5 days after sowing or Pretilachlor + safener (Sofit) 0.45kg/ha on the day of receipt of soaking rain followed by one hand weeding on 30 to 35 days after sowing.